1. Field of the Invention
The present invention relates to an amplification stage incorporating multiple power amplifiers having different input signals. The invention is particularly but not exclusively concerned with multiple power amplifiers in a transmitter of a MIMO system, for example in a cellular infrastructure base station or micro base station of a mobile communication system.
2. Description of the Related Art
MIMO (multiple input, multiple output) systems are well known in the field of mobile telecommunications. A MIMO transmitter generates multiple waveforms from a single signal source, which are amplified by multiple power amplifiers to provide multiple signal paths. The transmitted signals on the multiple paths are detected at one or more receivers.
It is well known in the art of RF power amplification to utilise envelope tracking modulated power supplies for power amplifiers in order to improve various parameters of the power amplifier and in particular the DC (direct current) to RF (radio frequency) power conversion efficiency.
The efficiency of a MIMO system can be improved by providing an envelope tracking modulated power supply for each individual power amplifier in the MIMO transmitter. The envelope tracking power supplies for each power amplifier are modulated so that the power supply voltage tracks the envelope of the input signal to be amplified. In a typical efficient arrangement one of a plurality of power supplies is selected in dependence on the input signal envelope, and then adjusted by an error correction means to more closely track the envelope.
Whilst the use of an envelope tracking modulated power supply provides an efficient amplification arrangement, it increases the overhead associated with the MIMO transmitter since an envelope tracking modulator stage is required for each power amplifier of the MIMO system.
It is an aim of the present invention to provide an improved arrangement for providing an envelope tracking modulated power supply in arrangements where multiple power amplifiers receive different input signals.